Sudo timeout driving me crazy. How can I extend the timeout?

I have read that after entering sudo followed by a command, and entering my password, the terminal should remember my password for 15 min so that I don't have to reenter my password for each command.

However, my sudo timeout seems to be zero. It makes me reenter the word sudo in front of every command line and reenter my password each time too.
How can I edit the sudoers file to set the time out to 15 minutes?